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[Caml-list] Polymorphic variants
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Date: 2002-04-17 (23:50)
From: John Max Skaller <skaller@o...>
Subject: Re: [Caml-list] Polymorphic variants
Remi VANICAT wrote:

>John Max Skaller <skaller@ozemail.com.au> writes:
>>Unfortunately, this doesn't work out as well as I'd expected
>>because of recursion:
>>type a = [`A of a  | `B]
>>type b = [`A of  b]
>>The second type is a subtype of the first,
>>but this fails:
>>let f (x:a) =
>>match x with
>>  #b -> ()
>>with the message:
>>This pattern matches values of type [< b] = [< `A of b]
>>but is here used to match values of type a = [ `A of a | `B];;
>the problem here is that you intended #b is a non fixed matching rule,
>I mean, imagine you have :
>`A (`A ( `A ( `A ( `A ( `A `B)))))
>to check that this is not in #b, one have to look deep in the
>structure of the value. 
Yes. But high level programming languages are *meant* to do
high level things, aren't they?

Here's another example that fails, which is very annoying:

type a = [`A of e | `B]
and e = [a  | `C]

"The type constructor a is not yet completely defined"

Arggg. Of course it isn't. It works just fine if I textually

type a =[`A of e | `B]
and e = [`A of e|`B|`C]

Almost all interesting cases of variants (for compiler writers anyhow)
involve type recursion. It shouldn't matter if the type is completely
defined, only that the definition is finally completed :)

BTW: I can't write  "t as x" in a type expression. Why not?
I'm forced to name it in a type binding .. that means that inductive types,
for example, can not be specified anonymously:

  (Cons of list | Empty) as list

Yeah, ocaml sum types are generative ... but the polymorphic
variant equivalents are not .. so

  [`Cons of list | `Empty] as list

should work. [Can I use a 'constraint' here? I've never used
a constraint before]

>pattern matching only look at the firsts
>levels of the structure. This must be done otherwise.
It doesn't "must" be done. I think you mean, there are advantages
only handling the easy cases: easy to write the compiler,
easy to optimise, easy for the end user to monitor
code complexity .. and easy for the programmer to handle
the more difficult cases manually ..

> for example :
>type a = [`A of a | `B | `C]
>type b = [`A of b | `C]
>let rec f (x:a) =
>  match x with
>  | `A x -> `A (f x)
>  | `C -> `C
>  | _ -> failwith "arg";;

The compiler doesn't know the type of x here,
at least without looking at the usage (clearly, usage
dictates type a). But the ambiguity could easily
be resolved by the client with an annotation.

>>No, I meant what I said! I can write both:
>>`A 1
>>and also
>>`A 1.1
>No, you can't. the types informations are lost at runtime, so ocaml
>will never be able to make the difference between the two.
I certainly can write them both:

`A 1; `A 1.1;
type int_t = [`A of int];
type float_t = [`A of float];;

No problem doing this. Perhaps there should be.
The problem only manifests here:

type num = [ int_t | float_t ]

because the same constructor name has two different
argument types. It is even possible to write a function
that accepts both

let f x = x

happily accepts both :-)

Now you say that the type information is lost at run time,
but that is NOT immediately apparent from the documentation.
Indeed, it need not be the case: the implementation *could*
mangle the constructor name with the argument type
so that


are treated as distinct constructors. Then the following:

fun x = match x with
| `A a -> ..

would give an error, since the compiler cannot know
which `A it is, however the problem could easily be fixed
by the user:

| `A (a:int) ->

now we know it is `A of int. Things "must be so"
if they are a consequence of theory, but it isn't
always evident which theory is used, and why that
one is best :-)

Perhaps one consequence of this is that the documentation
needs to be more specific and have more examples:
the author knowing the chosen theory and implementation
believes only lightweight coverage is necessary, not seeing
the many alternatives someone less involved might.

For example, C++ objects DO have associated and accessible
run time type information. I know Ocaml doesn't .. but there are
other things I know less well, and there are C++ users
coming to ocaml that might think pattern matching
uses RTTI.

To come back to the examples: the type

[`A of int | `A of float | `B]

is perfectly legitimate. The actual error is only on matching.
If the client never matches on `A, there is no error:

match x with | `B -> .. | _ -> ...

this match would be OK, `A isn't refered to.
It isn't clear why the compiler writer chose to allow
any argument of `A, but disallows specifying a name for
a type containing two distinct `A constructors instead
of simply reporting an ambiguity on use: they're
called "polymorphic variants" but they aren't actually polymorphic,
at least in this sense :-)

>it's not type in pattern matching, it's pattern. It is not, at all,
>the same thing, even if their is link between the two.
Sure, I know that. (Actually, the sum constructor tags really ARE
run time type information, they tell which case of the sum
is represented, and hence the type of the constructor argument)

>You should look to the mixin.ml file that can be found in testlabl
>directories of the source distribution, it show how to make what you
>seem to want.
The biggest problem I have with ocaml, is that while I can
make anything I want, it is often not evident just what the
compiler/language permits, and what it doesn't.

This is particularly serious for me when it comes to modules:
it just isn't clear to me what will work and what will not,
and experiments here can be very time consuming and laborious.
The ideas seem simple, but the syntax is sometimes quite arcane,
and all sorts of restrictions that don't exist in the underlying
basic category theory can bite you days into a project.

To give a more concrete example: the first time I tried to
use polymorphic variants, I lost a week and had to undo
all my changes because I kept getting error messages I
couldn't make head or tail of. I decided to try again
when my project got even more complex, and I really needed
stronger static typing, but didn't want to create a huge number
of standard sum types and laboriously code the maps between
them case by case .. not to mention the industrial requirement
that every type have an associated print routine, for example.

I pressed on despite the horrendous error messages,
knowing that the transormation HAD to be totally
mechanical in the first instance. [Replace A with `A ..]

I lost another week due to a bug in Ocaml 3.02.
It took a while to determine this really was a compiler bug
matching polymorphic variants (3.01 worked fine,
and so does 3.04).

[My point is that it is very hard to proceed when you don't
know if an error is a bug in your understanding, a bug
in your implementation, or a bug in the compiler ..
so it is safer to eliminate the first case by not using
advanced features .. but if this argument were extended,
it would be an argument for using C instead of ocaml...]

Well, having looked at the English translation of the 
Ocaml book by Emmanuel Chailloux, Pascal
Manoury and Bruno Pagano, I think a big hole is going
to be plugged. This book covers quite advanced topics,
I might learn something here!

Anyhow .. sadly, ocaml is such a wonderful language.
I say sadly because I am unemployed and poor .. and
just can't bring myself to work with C/C++ again.
Its just too much of a step backwards, and I'm not
enough of a diplomat to hide it from my fellow
workers or employers (who still think OO and C++
are wonderful, and Java is Gods own language ..)

John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.

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